A magnetoplumbite-type hexagonal ferrite including SrO.6Fe2O3 as a representative example, that is, an M-type ferrite, has previously remained a mainstream of ferrite sintered magnets. With regard to such an M-type ferrite magnet, efforts have been made to achieve a higher performance thereof, centering on the purposes that the grain size of a ferrite crystal is brought close to that of a single domain, that ferrite grains are aligned in a magnetic anisotropic direction, and that a high-density sintered body is achieved. As a result of such efforts, the properties of an M-type ferrite magnet are close to its upper limit. Thus, it is difficult to make a dramatic improvement of magnetic properties under the present circumstances.
W-type ferrite magnet has been known as a ferrite magnet, which possibly has magnetic properties that are far superior to those of the M-type ferrite magnet. W-type ferrite magnet has a saturation magnetization (4πIs) that is approximately 10% higher than that of the M-type ferrite magnet, and has the same level of anisotropic magnetic field as that of the M-type ferrite magnet. National Publication of International Patent Application No. 2000-501893 discloses a W-type ferrite magnet consisting of a composition represented by the composition formula SrO.2(FeO).n(Fe2O3), wherein n is between 7.2 and 7.7, and a sintered body thereof has a mean grain size of 2 μm or less and (BH) max of 5 MGOe or more. The aforementioned publication also describes that this W-type ferrite magnet is produced by steps consisting of (1) mixing SrO3 and Fe2O3 at a certain molar ratio, (2) adding C to the raw material powders, (3) calcining the mixture, (4) adding CaO, SiO2, and C to the resultant product respectively after the calcination, (5) milling the mixture resulting in a mean particle size of 0.06 μm or less, (6) compacting the obtained milled powders in a magnetic field, and (7) sintering the compacted body in a nonoxidative atmosphere.
Japanese Patent Laid-Open No. 2001-85210 discloses that a sintered magnet is comprised of a composite material formed by mixing one or two types of magnetite phases with a W-type ferrite phase, so as to obtain a ferrite sintered magnet having magnetic properties that are superior to those of a conventional M-type ferrite.
The aforementioned National Publication of International Patent Application No. 2000-501893 gives examples showing the achievement of a ferrite magnet having a saturation magnetization 4πIs of 5.0 kG. However, a ferrite magnet having a higher saturation magnetization 4πIs has still been required.
On the other hand, the aforementioned Japanese Patent Laid-Open No. 2001-85210 gives examples showing the achievement of ferrite magnets having a high residual magnetic flux density Br of 5.22 kG, 5.12 kG, or 5.06 kG. Japanese Patent Laid-Open No. 2001-85210 does not include any explicit descriptions regarding saturation magnetization 4πIs. In general, residual magnetic flux density Br is calculated in accordance with the expression “residual magnetic flux density Br=saturation magnetization 4πIs×magnetic orientation×density.” (It is to be noted that saturation magnetization 4πIs is calculated in accordance with the above expression in the present invention.) Accordingly, based on a conversion from the value of the residual magnetic flux density Br described in Japanese Patent Laid-Open No. 2001-85210, it is assumed that a saturation magnetization 4πIs of 5.44 kG or more was obtained in this publication. However, in Japanese Patent Laid-Open No. 2001-85210, such a high residual magnetic flux density Br was obtained in a mixed phase comprising a W-type ferrite phase and a magnetite phase (saturation magnetization 4πIs=6.0 kG). Taking into consideration the fact that such a magnetite phase is a soft magnetic phase, the method described in Japanese Patent Laid-Open No. 2001-85210 affects the squareness of a demagnetization curve in a BH curve. Squareness is also an important factor for magnets. Accordingly, even if a residual magnetic flux density Br and a saturation magnetization 4πIs are improved, it is considered that those having a low squareness are low in terms of magnetic properties that can actually be exerted.
Thus, it is an object of the present invention to provide a hard ferrite material or the like, which exhibits a high saturation magnetization 4πIs and a high residual magnetic flux density Br without impairing a squareness that is required for magnets.